1. Field of the Invention
The present invention relates to fabricating metal masks, especially molybdenum masks for microelectronic packaging.
2. Description of the Prior Art
Microelectronics uses crystals, usually of the semiconductor silicon. The crystals are sawed into "wafers," end processed by layering and diffusing various materials onto the crystal surface to form components such as transistors and resistors, which make up the circuits of the device.
The surface of a crystal wafer measures a few inches (up to 8") across, while the devices are only a fraction of an inch across. Each crystal wafer contains dozens to hundreds of copies of one device, laid out in a repeating pattern. After the devices are formed, the wafer is broken into rectangular pieces called "chips" or "dice" (singular "die"), with each die containing one device.
To be useful, a die must be electrically connected to other electronic devices. This connection cannot easily be made by directly wiring the dice together. Instead, the dice are "packaged" in units. A package includes conductive leads that extend from the outside of the package to metal pads on the die.
Inside the package, the die is glued onto a substrate, which might be made of various materials. The substrate material is desirably mechanically strong and rigid to support the dice, desirably an insulator so that the electronic currents flowing through the package will not leak away, and desirably expands with rising temperatures in about the same way as silicon does. If the dice expanded and shrank relative to the substrate, they might break off after too many heating and cooling cycles. Substrate materials include ceramics such as alumina, plastics such as polyimide, and silicon crystals which are highly purified (and so not very conductive, unlike doped silicon).
After mechanical mounting, electrical connections are made from the die to the prongs, pins, or pads on the outside of the package.
A microelectronic circuit's speed and cost are directly related to its size, so the components on a chip are made as finely as possible. Chip component sizes have decreased at about ten percent per year due to engineers' continuing efforts to make the components as small as possible. Since the chips must also communicate, package sizes may also be small. Packages take up a large share of the processing time delays and the cost of electronic devices. Small, but dense, packages are commercially important.
A method in the prior art of connecting a die to the outside leads was by soldering very fine gold wires between the leads and the die pads. A more recent and efficient method, Tape Automated Bonding (TAB), uses a sandwich of insulating plastic and patterned conductive molybdenum foil laid out in strips that act as wires. Each die pad has a tiny solder bump. The foil pattern is aligned to the die pads so that the end of each strip of molybdenum is above a pad solder bump. The foil is pressed onto the chip, and the solder bumps connect the foil strips to the pads, making all the connections at once. TAB allows pads to be spaced only 0.25 mm apart around the perimeter of the chip package.
Molybdenum has been used because its coefficient of thermal expansion is about the same as silicon's coefficient of thermal expansion. When the temperature varies, both molybdenum and silicon expand or shrink by the same amount. This prevents misalignment of the pads and foil strips. Molybdenum also has good mechanical properties and conducts heat well.
Molybdenum's mechanical strength, convenient thermal expansion coefficient, conductivity, and ability to withstand high temperatures make it useful for other applications in microelectronics technology. One such application is "moly masks," sheets of molybdenum foil used as templates for various microelectronic fabricating operations.
Moly masks are used with Controlled Collapse Chip Connection (C4) packages. C4 is also known as "solder bump" and "flip chip." In C4, the entire surface of the chip package is covered with conductive pads. The package is pressed down onto a substrate or PC board with corresponding pads having solder bumps or balls, partially collapsing the solder balls and making connection between the respective pads. C4 allows a high density of electrical interconnections. It differs from the TAB system in that the pads cover an entire side of the chip, rather than the perimeter.
The type of moly mask used for C4 is a "device mask" or "evaporation mask," a sheet with up to 100,000 holes through it. Each hole, or "via," is about 4 thousandths of an inch in diameter. The vias in the mask are used to form the tiny solder bumps in place.
In a high-vacuum chamber, the mask is placed over a silicon wafer in precise alignment with the components on the wafer surface. Solder is evaporated above the mask, and the metal vapor comes through the vias and condenses on the wafer surface to make the solder bumps. The vias on the moly mask are desirably located to a precision of plus or minus 4 ten-thousandths of a inch, or the terminal metal may condense in the wrong places and ruin a very expensive wafer. When heated, the molybdenum expands along with silicon, thus keeping the pads and vias in registration.
Another type of moly mask, used for making multi-layer ceramic (MLC) substrates, is a "screening mask." The vias in a screening mask serve as a mold through which screening paste, made of molybdenum powder and binder, is squeezed onto selected areas of a substrate. The mask is laid onto the substrate and paste is extruded into the mask holes from a nozzle passed over the mask. Excess paste is removed from the mask surface. The substrate is then fired in an oven to drive off the binder and to sitter the powder into conductive metal. The areas between conductive metal regions are filled with a non-conductive glass or ceramic material.
This process can be repeated on a substrate, using a series of different masks, to form a stack of layers. The masks are designed so that the conductive metal patterns interconnect between layers an selected points. The whole stack thus forms a complex circuit for electrically joining various chips mounted on the substrate surface.
The MLC process cam be applied to substrates as large as 127 mm (5 inch) square, containing thousands of vias and lines in degrees of layers and containing up to millions of connections.
The vias in a screening mask may be only on the order of a thousandth of an inch in diameter, so the dimensional stability of screening masks is a critical criterion.
A prior art method of making moly masks includes the use of photolithography and chemical etching on molybdenum foil. This technique is well known to one skilled in the art.
One solution commonly used to dissolve the molybdenum foil is alkaline potassium ferricyanide. It is often sprayed onto the mask surface, as is discussed in The Principles and Practice of Photochemical Machining and Photoetching, by D. M. Allen, published by Adam Hilger, Boston (1986).
Potassium ferricyanide and other cyanide solutions which are used for etching are highly toxic. Furthermore, cyanide in the form of fumes or discarded chemicals is an environmental pollutant. In the work place, cyanide fumes are especially insidious because cyanide dulls the sense of smell. Workers can be exposed to increasingly concentrated poisonous fumes while believing that the fumes are vanishing. Cyanide has recently been banned in New York State.
Besides the danger and pollution of cyanide etching, the process has other drawbacks. It is a slow process, which means that a large capital investment and much cyanide are needed to produce masks at a certain rate. Another problem is the limitations on the "aspect ratio" of the openings in the moly mask foil, the ratio of opening width or via diameter to foil thickness.
The cyanide etch does not bore straight into foil like a drill. The etching is isotropic and acts on all exposed surfaces, and the cyanide eats away the molybdenum under the edge of the photoresist. This undercutting limits the aspect ratio.
Undercutting increases the size of mask holes and slots beyond what was intended. It also blurs the sharp outlines defined by the photoresist edges and limits the closeness of vias or other features.
Since the mask is left in the cyanide long enough to dissolve the foil thickness, the cyanide will have a roughly equal time to dissolve under the edge of a photoresist ledge. Thus, to a first approximation, the undercut width will be of the same order as the thickness of the foil. However, the foil cannot be made too thin or it will lack mechanical strength.
The undercutting resulting from the cyanide process limits the fineness of the mask pattern and prevents moly masks from being used with small microelectronic components. Any decrease in undercutting would be a commercial advance.
Several drawbacks of chemical etching can be overcome by substituting electrochemical etching, in which an electrolyte (salt solution) and electricity are combined to etch metal. The etched metal is called an anode. Another piece of metal, called the cathode, is immersed in the salt solution along with the anode, and a voltage is maintained between them. The combination of electric current and electrolyte gradually etches the anode, which dissolves away. The salt solutions used are harmless and the voltages are low, so the process is safe and non-polluting.
One form of electrochemical etching is electrochemical machining (ECM). ECM employs a having a certain desired shape, a fluid electrolyte, and intense electric currents between the tool and a metal workpiece. ECM forms a depression shaped like the tool in the workpiece. During the machining operation electrolytic fluid is continuously pumped at high rates through the small gap between the tool and workpiece.
FIG. 1 illustrates a basic electrochemical etching cell. A tank T holds liquid electrolyte E, an aqueous solution of a salt (for example, table salt and water). The anode A and the cathode C are wired to a voltage source such as a battery B. When the apparatus is electrified, metal atoms in the anode A are ionized by the electricity and forced out of the metal into the solution, so that the metal dissolves in the water. The rate of dissolution is proportional to the electric current, according to Faraday's Law. Depending on the chemistry of the metals and salt, the metal ions from the cathode either plate the cathode, fall out as precipitate, or stay in solution.
Electrochemical machining (ECM) is based on the basic electroetching set-up of FIG. 1. In conventional ECM, the cathode is a shaped tool which is held close to the anode and slowly moves toward it. The anode is the workpiece, which is machined away as it dissolves. A variation of ECM is resist pattern ECM or electrochemical micromachining (EMM), in which a resistant layer is adhered to the surface of the anode. Etching takes place in between the resist layers on the bare metal, without any need to move the cathode or maintain a close tolerance on the anode-cathode distance.
U.S. Pat. No. 4,212,907, issued to Ralph Wright, teaches a method of treating the surface of molybdenum prior to depositing a nickel-phosphorus coating. The surface is anodized in an acidic medium to form a film of gray molybdenum oxide, and then the film is removed. The treatment improves the adhesion between the molybdenum and the nickel, and is useful in making laser components.
James McDavid teaches the use of molybdenum masks in the fabrication of metal-gate semiconductor devices in U.S. Pat. No. 4,628,588. His invention addresses the problem of removing molybdenum oxides from a sidewall of an oxide/molybdenum stack. His method first builds layers of refractory material, insulator, molybdenum, and patterned photoresist, then etches through the molybdenum using the photoresist as a mask, and finally etches the insulator using the etched molybdenum as a mask.